The present invention relates to an air-fuel ratio control method and apparatus for an internal combustion engine.
It is known practice to provide an internal combustion engine with a closed-loop system for controlling the air-fuel ratio, which calculates an air-fuel ratio correction coefficient f(A/F) responsive to detection signals fed from a concentration sensor which detects the concentration of a particular component contained in the exhaust gases, such as from an oxygen concentration sensor (hereinafter referred to as O.sub.2 sensor) which detects the concentration of oxygen in the exhaust gas, and which corrects the amount of the fuel injected into the engine relying upon the calculated coefficient. In the internal combustion engine of this type, the air-fuel ratio correction coefficient f(A/F) is fixed to a predetermined value when the engine is under predetermined operating conditions, and the function of closed-loop control is discontinued. For example, the air-fuel ratio correction coefficient f(A/F) is maintained at a constant value irrespective of the detection signals of the O.sub.2 sensor when the coolant temperature of the engine is lower than a predetermined value, or when the opening degree of the throttle valve is greater than a predetermined value and thus the rate of feeding the fuel is additionally increased, or when the O.sub.2 sensor is inactive, or when the supply of the fuel has been cut off. Accordingly, closed-loop control of the air-fuel ratio, relying upon the detection signals of the O.sub.2 sensor is discontinued (the period when closed-loop control has ceased to work is hereinafter referred to as the period of open-loop control). According to the conventional art, the initial value of the air-fuel ratio correction coefficient f(A/F) when closed-loop control is started again after open-loop control is finished, is set to be equal to a fixed value of the air-fuel ratio correction coefficient f(A/F) during open-loop control. Namely, the initial air-fuel ratio correction coefficient f(A/F) when closed-loop control is resumed is selected to be equal to an air-fuel ratio correction coefficient f(A/F) at closed-loop control just before open-loop control, or to be equal to a predetermined value, for example, equal to f(A/F)=1.0.
According to the above former method, however, the initial air-fuel ratio correction coefficient f(A/F) when closed-loop control is resumed is greatly deviated from an optimum value of f(A/F) if closed-loop control just prior to open-loop control was carried out under very particular operation conditions. Thus the coefficient f(A/F) requires considerably extended periods of time to reach an optimum value. Even with the above-mentioned latter method, the optimum coefficient f(A/F) is often greatly derivated from the initial coefficient and thus extended periods of time are needed until the coefficient f(A/F) reaches an optimum value. Furthermore, in case where a so-called rich monitor control is effected, i.e., where closed-loop control forcibly stopped when the coefficient f(A/F) charges by more than a predetermined value over a predetermined period of time, it often happens that the coefficient f(A/F) is not at all permitted to reach the optimum value.
If the coefficient f(A/F) is not allowed to readily reach the optimum value or is not at all allowed to reach the optimum value even after closed-loop control has been resumed, performance for controlling the air-fuel ratio at a desired air-fuel ratio is deteriorated as a matter of course, and the operating characteristics of the engine are deteriorated. Furthermore, the function for purifying the exhaust gas is diminished, as well.